The conventional way of making such perforated foils involves the application of a photosensitive layer to a metallic substrate and the selective removal of certain parts of that layer by a development process after exposure to illumination through a mask bearing the desired pattern. The portions of the layer remaining on the substrate form a pattern of isolated dots around which a coherent metallic layer (e.g. of copper) can be built up by electrolytic deposition. When this metallic layer is stripped off the substrate, and upon removal of the dots of photosensitive material (which could take place either before or after the stripping step), it is ready for use in screen printing.
In commonly owned U.S. Pat. application Ser. No. 781,063 filed by Siegfried Ruckl on Mar. 24, 1977, now U.S. Pat. No. 4,118,288, there has been disclosed and claimed a process of this general type designed to obviate a drawback of the aforedescribed technique, namely the fact that the electrolytically deposited metal tends to grow laterally into areas occupied by the dot pattern so that the resulting perforations in the foil are smaller in diameter than the original dots. The process of that application and patent involves an increase in the porosity of the photosensitive-layer portions left on the substrate, allowing these portions to swell upon being contacted with moisture (e.g. during the developing step) so as to reduce the intervening interstices to be occupied by the galvanically deposited metal.